The present invention deals generally with measurements of the roll rate and roll angle of spinning platforms, including spinning projectiles, spin stabilized spacecraft, and other such vehicles, using a single receiver antenna for receiving signals transmitted by the satellites of the Global Positioning System (GPS).
In the context of the invention herein, the terms “roll” and “spin” are used interchangeably and understood to mean the platform's rotation about its “roll” or “spin” axis at angular rates that exceed the zero roll rate and may be large. The words “platform,” “vehicle,” and “projectile” too are used interchangeably in this specification and must be interpreted as inclusive, so that the mention of one also means the mention of the others and similar like terms. The term GPS also is to be construed broadly, and includes not only GPS but all Global Navigation Satellite Systems (GNSS) using CDMA (Code Division Multiple Access) technology.
The focus on roll results from the fact that many projectiles aimed at a distant target do not require a full attitude reference system. If the projectile is stable under all flight conditions encountered, it may not require any stability augmentation about its two cross axes, pitch and yaw. Every guided platform, however, requires a measurement of its roll angle or roll rate, since this information helps relate the guidance commands, which are executed in the platform's body coordinates, to its location in space, which is identified in geographic coordinates.
The requirement for roll information on a guided projectile differs with the projectile configuration and its guidance concept. A fully controlled projectile is one that can correct its trajectory in both the downrange and cross-range directions. To do so, it must be able to generate lift in both the upward and lateral directions. This requires knowledge of which way is up and which way, for example, is to the right. The guidance corrections are relatively insensitive to the vehicle's elevation and azimuth Euler angles, but very sensitive to its roll angle. Reasonably accurate roll angle information is required. Since the command to lift the spinning projectile in a particular geographic direction is transformed into the projectile's body coordinates for execution by its control actuators, such as aerodynamic fins, this means that a fully controlled projectile—or at least that section of it that houses the control system—cannot be spinning faster than the bandwidth of its actuators. Other methods of making downrange corrections to the platform's trajectory include adjusting the drag on the projectile.
Limited corrections of cross-range may be achieved by adjusting the projectile spin rate, which leverages the spinning projectile's natural tendency to drift in the lateral direction. This configuration requires a relatively high spin rate, typical of gun-launched projectiles. But these corrections do not require roll angle information only roll rate is needed. So the usual roll information requirement of the roll estimation system is either for roll angle (at relatively low spin rate) or for roll rate (at relatively high spin rate).
Traditional methods of measuring roll rate and roll angle are expensive, and can generally be justified only for very high value platforms. Inertial rate indicators, including MEMS (MEMS=Micro Electro Mechanical Systems) gyroscopes, are relatively expensive on this scale of costs. More importantly, they require calibration prior to use, which adds to the operational cost. Magnetometers, likewise, are expensive, besides also being susceptible to interferences from local magnetic fields, such as from on-board electromagnetic actuators and other components.
For cost-effectiveness, GPS-based measurements are preferred for determining roll and attitude of rotating platforms. GPS carrier phase and signal strength measurements are the two main techniques used for attitude determination.
Prior art roll measurement techniques have largely employed phase differencing of GPS signals received at a plurality of receiver antennas mounted on the spinning platform. The different phases of the signals at the different antennas arise from the different distances of those antennas from the signal source, i.e. the satellite. Most of the prior art baselines (i.e. the distances separating the individual antennas from each other), however, have typically been long compared to the GPS signal wavelength. This has required keeping accurate count of the number of integer wavelengths in each of the received signal paths (in addition to the fractional wavelength determinations) for calculating the true path lengths (and hence phases) of the signals reaching the respective antennas. In the commonly owned U.S. Pat. No. 7,994,971, Vander Velde et al have disclosed a system and method for measurement of roll rate and roll angle that dispenses with this requirement by using a plurality of receiver antennas with baselines shorter than the GPS signal wavelength. Separately, in another co-pending and commonly owned U.S. patent application Ser. No. 12/454,306, Vander Velde et al have disclosed a system and method to measure roll rate and roll angle by such plurality of receiver antennas in the presence of interfering signals.
Roll determination using a single receiver antenna has the potential for even greater simplicity and cost effectiveness. For example, U.S. Pat. No. 6,520,448 B1 2/2003 issued to Doty et al discloses a single antenna based method and apparatus for roll measurement using amplitude and phase detection of the received GPS signals. When the platform rotates, the GPS signal received at the single antenna shows time varying characteristics, which provides information for roll determination. The power or carrier phase of the signal received at the single antenna from the GPS satellite shows a modulation over the antenna's spin cycle, whose period is a measure, of the vehicle spin rate. The power of the received signal is maximum at the roll angle which orients the antenna boresight nearest to the Line of Sight (LOS) to the satellite. Since the direction to the satellite is known in Earth-fixed coordinates, this determines the roll angle of the platform. Modulation tracking, as used in Doty et al, however, is very sensitive to noise, and small amplitude and/or phase measurement fluctuations cause large variations in the measured roll angle and roll rate values. Another limitation of tracking amplitude or phase modulation, as in Doty et al, is that the tracking loop bandwidth is difficult to tune for an accurate roll estimate over a large variation of roll rate (e.g., up to 300 Hz or more for fast spinning projectiles).
To operate over large ranges of roll rate and its time derivatives, U.S. Patent Application Pub. No. 2010/0052981 of Alexander et al uses a fourth (4th) order Phase Lock Loop (PLL) in the power (i.e. amplitude) modulation tracking filter. The PLL locks on to the phase of the amplitude modulation resulting from the projectile's spin. A roll compensator is used to compensate the PLL output for roll angle-dependent changes in antenna pattern gain and phase, which can lead to phase mismatch between the actual antenna pattern phase and its PLL estimate. A Kalman filter is used to smooth the un-modeled errors in the estimates of the “up” direction for each satellite tracked by the single antenna. These extensive signal processing methods for addressing model mismatch and un-modeled errors are required because the approach disclosed by Alexander et al is non-optimal.
An easily implemented and cost-effective optimal solution of low complexity is desired for accurately determining in real time the roll rate and roll angle of low cost spinning projectiles, with several spinning at rates of up to 300 Hz or more. For projectile roll rate and roll angle measurements using multiple receiver antennas, such solutions based on phase and/or amplitude differencing were disclosed recently in the commonly owned U.S. Pat. No. 7,994,971 and U.S. patent application Ser. No. 12/454,306 (published as US 2010-0289687). The present invention builds upon these earlier developments, while using the output of a single antenna in a manner that avoids the limitations of the prior art and provides an accurate real time measurement of the projectile's roll rate and roll angle at projectile spin rates of up to 300 Hz, and likely higher.
An important property of the system and method employing the roll filter based roll processor described herein is that it needs to track only one satellite for providing the roll rate and roll angle information. However, the present invention is able to accept amplitude and/or phase measurements from additional satellites, if available, for improved accuracy.
Although the instant roll processing methodology can function using only a single satellite, the roll information is usually used in conjunction with the vehicle's navigation (position and velocity) information in order to execute a task, such as guiding the vehicle to a target. Where such position and velocity are to be provided by a GPS receiver, a minimum of four satellites must be tracked for that purpose.